Photoreceptors, particularly those related to xerographic copying, traditionally comprise a photoconductive insulating layer such as an element or alloy thereof exemplified by selenium (amorphous or trigonal) and selenium alloys such as a Se-As, Se-Te, Se-Bi, etc., with varying amounts of a halogen. These photoconductive materials are customarily applied in charge blocking contact to a supporting metal- or metal-covered substrate such as aluminum, steel, nickel, brass, NESA glass or corresponding metal-coated polymeric materials.
Functionally speaking, xerographic photoreceptors utilizing the above elements are generally given a uniform electrostatic charge and the sensitized surface exposed to an image pattern defined by an electromagnetic radiation, such as light. Light impingement effects the selective dissipation of the initially applied charge leaving a positive electrostatic image. The resulting (latent) electrostatic image is then developed by applying oppositely charged marking particles onto the charge-bearing photoreceptor surface.
The basic xerographic concept was originally described by Carlson in U.S. Pat. No. 2,297,691, and has been since amplified and redescribed in many related patents in the field. Generally speaking, however, photoconductive layers suitable for carrying out the above functions have a specific resistivity of about 10.sup.10 - 10.sup.13 ohm-cm, in the absence of illumination. In addition, their resistivity must drop at least several orders of magnitude where exposed to an activating radiation such as light.
Photoconductive layers meeting such criteria are expected to exhibit some loss in applied charge, even in the absence of light exposure. This phenomenon, known as "dark decay", unfortunately usually varies inversely with sensitivity and in direct proportion with usage of the photoreceptor. The latter characteristic is generally attributed to a natural tendency of selenium to convert from an amorphous to the crystalline (trigonal) form with passage of time and exposure to temperatures exceeding a predetermined range.
Control over the rate and conditions effecting such conversion, and its affect, is a matter of primary concern, particularly in view of the fact that modern xerographic photoreceptors have rather stringent demands with regard to increased spectral response and general photoconductive efficiency as well as good mechanical properties such as flexibility and durability. This has become particularly important in modern automatic copiers operating at high speeds where the photreceptor is in the form of an endless flexible belt (ref. U.S. Pat. No. 2,691,450). For example, high speed machine cycling conditions require particularly strong adhesion and durability between the photoconductive layer and the underlying substrate. Unfortunately, however, the most sensitive and efficient selenium photoconductive materials are relatively brittle and do not generally adhere well to a flexing metal substrate. It is also found to be very important that any interface between an electrically conductive supporting substrate and the photoconductive layer be chemically stable as well as strongly adherent since changes at this point will have a substantial effect on the electrical properties of a photoreceptor including dark decay.
In practice, the present or potential technical difficulties inherent in the modern usage of selenium have been solved only in part and by a trade off or compromise in properties.
First and foremost, hole-generating photoconductive layers, such as trigonal selenium, can be easily utilized in photosensitive members having a base, an imposed charge generating photoconductive layer and an external hole-transporting overcoat layer for selective discharge of the surface (sensitizing charge) in conformity with the area of light exposure.
The use of such material, however, tends to cause undesirable dark decay problems. A partial answer, in the more conventional xerographic mode, has been provided by incorporation of a thin barrier layer such as a dielectric film between the base or substrate and the photoconductive layer. U.S. Pat. No. 2,901,348 of Dessauer et al, for instance, utilizes a film of aluminum oxide of about 25 to 200 Angstrom also insulating adhesion-promoting polymeric resin layers, such as a polybenzimadazole, polycarbonate, polyester, polyurethane or polystyrene of about 0.1 to 3 microns thickness for such purpose. With some notable limitations, such barrier layers function remarkably well and allow many types of photoconductive layers to support a substantial charge. When activated by illumination, however, the photoconductive layer and barrier layer must also operate together to permit substantial dissipation of the applied charge in light-struck areas within a relatively short period of time and also provide a reasonably good spectral response range.
In addition to other needed properties the amount and type of alloy additives, such as arsenic, etc. are found to substantially stabilize or limit the rate of conversion of amorphous selenium to the crystalline trigonal form. Depending upon the type of device desired and its problems, however, it is sometimes necessary to have both the crystalline and non-crystalline form present initially as part of the photoconductive or charge-generating layers. On balance, therefore, flexibility and control over the form and type of charge generating layer to be laid down and stability are greatly to be desired.
It is an object of the present invention to obtain a process or method whereby charge-generating selenium or selenium containing photoconductive layers can be selectively and efficiently laid down to obtain desirable combinations of physical and electronic properties.
It is a further object of this invention to improve the flexibility and overall responsiveness of selenium-containing xerographic photoconductive material to the challenges of modern xerographic copying.
It is a still further object of the present invention to easily and efficiently control the initial amount of photoconductive crystalline selenium initially applied onto a base or substrate during coating operations and to maintain a stable relationship between crystalline and non-crystalline material deposited.